Process for the production of high alpha-cellulose fiber



A G1. A. RICHTER PHOcEss FOR THE PRODUCTION OF HIGH ALPHA OELLULOSEFIBER Jan. 27, 1931.

Original Filed May 6 w15 ai w w 1 n W u Patented .Isa 27, 1931 l UNITEDSTATES PATENT oFFlcE GEORGE A. RICHTER, F BERLIN, NEW HAMPSHIRE,ASSIGNOR TO BROWN COMPANY 0F BERLIN, NEW HAMPSHIRE, A CORPORATION OFMAINE PROCESS FORLTHE PRODUCTION OF HIGH ALPHA-CEIBLULOSE FIBERApplication led Hay 6, 1925, Serial No. 28,339. Renewed July 23, 1927.

This invention relates to the utilization of the heat and the recoveryof spent sodium digesting constituent fromv the spent liquor of analpha-cellulose liber plant operated simultaneously or in connect-ionwith a kraftor sodium sulphate plant.

High alpha-cellulosefiber may be produced by an alkaline digestion of,for instance, unp bleached sulphite pulp,--a caustic soda, soda 4 verywhite color /after bleaching. The alka-v line digestion may take placein digesters at any suitable temperatures and pressures, although I havefound it in many cases to be more economical and expedient to digest thesulphite pulp at atmospheric pressure inopen tanks under heat. In order4to render the process more economical, the spent sodium constituentshould be recovered and the available heat contentof the spent liquorshould be utilized.

In my (fo-pending application Serial No. 26,361, filed April 28, 1925, aprocess for the recovery of the sodium constitue-nt and the utilizationofthe available heat content of the spent digesting liquor has beenoutlined. ASuch a process, however, requires a special 35 recoveryplant, where it is practiced in a plant wherein sulphite pulp only isproduced. The objectof this invention is to provide a. method forpracticing the economies set forth in the co-pending application, whilecombining the recovery of sodium constituent from the spent liquor in aualphascellulose plant with the recovery of sodium constituent from. thespent black liquor o" a kraft plant, thereby eliminating the necessityfor special recovery apparatus in the alpha-cellulose plant. Thisinvention may best be understood from the following descriptionthereofwhen taken in connection with the accompanying drawing, whichrepresents what may be termed a flow sheet. ff-

viously a continuous cyclic process is preferable.

Before proceeding to the features of invention involved herein, theoperation Aof the alphaplant, which has been fully described in theco-pending application, will be brieliy n reviewed.

Sulphite pulp is produced, as usual, by the digestion of Wood chips (e.g., spruce) in a digester, in a suitable sulphite liquor, as, forexample, calcium bisulphite digesting liquor. After the necessary periodof digestion, the digester contents are blown, and the sulphite pulp issuccessively Washed, screened, and thickened to, say, a consistency ofabout 50%, as by press rolls. The thickened, -unbleached sulphite stockthus obtained is continuously fed into a digesting tank A, and isdiluted to a consistency of about 10% with water and the amount ofcaustic soda or soda ash solution required for the purification of thesulphite pulp. The diluted sulphite pulp is continuously delivered yintothe bottom portion of a second digesting tank B, from the tank A, at thesame rate as it is being madev up in the tank A, by a slow-moving wormconveyor, and overliows from the second tank B. TheA stock is digestedand slowly agitated yduring its passage through the tanks A'and B at 180F; to 212 F., by blowing steam at suitable points .into thesuspension,-the lessresistant celluloses and the ligneous and othercoloringv matters contained in the sulphiteA pulp reacting with andbeing dissolved by the alkaline solution so that the product whichleaves the top portion of the second tank B is high in alpha orresistant cellulose. The agitation rovided for during the digestion andthe eed of the stock by the Worm from one tank to the other givesintimate contact between the stock and the liquor, and thereby promotesa thorough reaction between the sulphite pulp and the alkalineliquor,resulting in a uniform high alphacellulose pulp product. Of course, thedigestion may be carried out as a batch process, but ob- The stock,together with the spent digesting liquor associated therewith, leavingthe top portion of tank B, and now containing a high percentage ofalpha-cellulose, is passed 10o 'through a counter-current washer,wherein the stock is thoroughly washed free from substantially all thedigesting liquor, and the washed stock is then passed to the bleachery.The Wash Water used in the washing apparatus assumes practically thesame tem erature as the hot spent digesting liquor an the hot pulp asthey are passed in the washing apparatus. The hot wash water (which forconvenience of designation I term the alpha spent liquor") containssubstantially all the sodium constituent in any form, spent orotherwise, associated with the alpha stock as it was passed into thewasher. The alpha spent liquor is re-circulated to the digesting tank A,and comprises in volumelapproximately the volume of water associatedwith the pulp after digestion, plus the total volume of wash water usedin the Washer, minus that portion absorbed by the washed alpha pulp asit passes to the bleachery; During this stage of the process, the volumeof this` wash water employed in washing the digested sulphite pulp is soregulated that the volume of spent alpha liquor obtained from the washerand re-circulated into the tank A is substantially equal to the volumeof Water and digesting solution employed in making up the initial 10%stock suspension in the tank A. To this re-circulated wash water iscontinuously added a sufficient quantity of sulphite pulp and the amountof concentrated caustic soda or soda ash solution necessary to removethe less-resistant celluloses and the ligneous matter contained in thesulphite pulp to produce a stock suspension of about 10%. The new 10%stock suspension formed with the re-circulated wash water is digested,and the cycle of operations as hereinbefore described in connection withthe initial 10% stock suspension is repeated. The hot spent liquorobtained from the washer at this point in the process contains a higherper cent of dissolved material. This spent liquoris recirculated asbefore and the process as describedis continued or repeated until a hot,syrupy spent-liquor of about 10 to 15 Baum results.` Such a concentratedspent liquor may profitably be evaporated and the combustible contentthereof burned to recover-the valuable sodium constituent.

lYhen this concentration of spent liquor is ariived at, the flow of-`fresh water to. the

washer is increased. As stated previously, up

- stituent is taken into account; but there is now an excess of spentliquor. This eiessjs continuously withdrawn from the spent liq- A uorobtained from the washer, and mixed with the black liquor -resultingfrom a kraft or sulphate pulp digestion as it is passed to theevaporators. The rest of the spent alpha liquor is re-circulated intothe alpha. digesting tank A. The increase in quantity of wash water,thatis, the excess passed along with the black liquor to the evaporators, isso adj usted as to maintain an equilibrium within the system at thisparticular concentra-l tion: that is, the excess spent liquor mixed withthe spent kraft liquor contains substantially the amount of spent sodiumconstituent which was formed in the preceding digestion (the term sodiumor sodium constituent as used in thisl specification and the appendedclaims will, of course, be understood to mean sodium in combined form,that is, in the form of a sodium compound).

As indicated on the drawing, it may be stated that the black spentliquor hereinbefore referred to results from the digestion of wood chipsin a digester in the usual kraft or sulphate digesting liquor, whichcontains essentially caustic soda. sodium sulphide, together with asmall quantity of sodium carbonate. After digestion, the digestercontents are blown and the kraft pulp is passed through acounter-current washer similar to the one in which the alpha-cellulosepulp is washed. The washwater from the washer contains substantially allthe di esting constituent associated with the kra t pulp as it is passedinto the washer, is black and of about a 14 Baum,and is known as spentkraft liquor. In some cases, it may be desirable to re-circulate oneportion (usually about one-half the total of such spent liquor to thekraft digester) for use together with newly made digesting liquor in asubsequent kraft digestion. The other portion of black kraft spentliquor is passed into a storage. tank,

from which it is continually withdrawn to the evaporators. To this blackliquor, as has been previously stated. is added the excess spent liquorof from 10 to 15 Baum, obtained from the alpha spent liquor, so that amixture of alpha and kraft liquors of about the same degree Baume ispamd in to the evaporators for concentration. The resulting concentrateis burned, preferably in a' combined boiler and smelting furnace, in areducing atmosphere. Additional sulphur may be added as described inLetters Patent No. 1,469,960, dated October 9, 1923,-to the mixedconcentrated liquor,-the amount of sulphur added thereto being regulatedand controlled so as to form a smelt of approximately the samevcomposition asV the usual kraft smelt. The ciuent smelt from the furnaceis caught in a relativelysmall quantity of water in a dissolving tank,and a concentrated solution is formed which contains essentally sodiumcarbonate, sodium sulphide, and relatively small quantities of unreducedsodium sulphate .and sodium hydroxide.

This solutionis then causticized with lime,

' resultant white4 sulphide liquor, which contains sodium hydroxide,sodium sulphide, and smaller quantities of unreduced sulphate andunconverted sodiumcarbonate, is passed to a storage tank.

This strong white liquor may furnish a portion of the alkalinedigesting, agent re-v quired in the alpha plant tanks. This portion ispreferably such that, together with thel unspent alkali present in there-circulated portion of spent alpha liquor, it contains the totalalkali necessary for the alpha digestion, minus the losses suffered inboth the alpha and kraft pulp cycles.. From the foregoing statement itis obvious that if the alpha pulp and kraft pulp. production happen tobe in a certain proportion; that is, when the fore- 1 going differenceis zero or a minus quantity,

no white -liquor will be supplied to the alpha pulp digesters, inwhichcase the alkali-necessary for the alpha digestion is supplieddirectly to the digesting tank A, as shown. To

understand this Vmore clearly, the following example will suiice.

Thesodium sulphate losses in a very efficient 'kraft plant amount toabout 220 pounds of salt cake (sodium sulphate) per ton of pulpproduced. .The salt cake required to 'make up these losses is equivalentto about 125 pounds of caustic soda,"and would repreduction of highalpha pulp. Assuming, then,

that the alpha plant and the kraft plant are producing the samequantities of pulp, the

f spent liquor recovered' from the alpha-cellusay, the total' recoveredsodiumin the mixed spent liquors would'contain just the correct l amountof sodiumconstituent for the next digestion in the kraft plant, andunder such `conditions no white liquor would be delivered `to the alphadigesting tanks. In other Words,

Y the recovered sodium constituent from a ton i of alpha pulp wouldserv@l as a make-up niaducing a tonvof kraft terial for the sodiumlosses suffered in propulp, andin J4"such a case itwouild be necessaryto add. to the alpha tanks the total sodium losses suffered in both Y'cycles, which would be equal to the amount of sodium constituentrequired for digesting the unbleached sulphite ulp.-

In actual practice, owever, it is quitelikely that'such a balance couldnot be' n'1ain'4 tained, since the quantity of alphapulplpro ducedVmightA be slightlyin excess of. the

the next kraft digestion, and this excess4 70 would be returned to thealpha digesting tanks. In addition to this returned White liquor, inorder to maintain the system at equilibrium, an amount of alkali equalto the kraft and alpha cyclelosses is added to the. alpha digestingtanks, and the entirecycle may be repeated. 0r, if the quantity of kraft.pul produced is slightly in excess of the alp a pulp production, noWhite liquor would be delivered to the alpha digesters. In'this 80 case,the amount of alkali Which it would be necessary to introduce into thealpha digesters if the production of both the alpha and kraft plantswere equal is then ladded to the alpha digesters. The kraft cycle lossesnot made i of sufficient sodium constituent in the form ofsodium-sulphate, or in the form of sodium carbonate, caustic soda, andsulphur, to the mixed.. spent liquor so as to give the usual kraftsmelt.

Under conditions when no alkaline liquor is available from the kraftmill, then the unbleached sulphite pulp is digested entirely in causticsoda digesting liquor. If the alpha fiber plant uses availableliquorfrom the i kraft mill, there are present in the -liquor a certainamount of sodium'sulphide and a relativel small amount/of otherconstituents, v10e so that t e sulphite pulp is digested in a mixture ofcaustic soda and sodium sulphide. Such a liquor produces as satisfactoryan alpha pulp as isproduced with caustic soda solution alone. liquor maybe a solution of sodium carbonate or of a` mixture of sodium carbonateand caustic soda, or of a mixture of sodium car bonate, sodiumhydroxide, and sodium sulphide, or any other combination of thesecon-v'. 110 stituents.

Aside from the advantages outlined in the l co-pending application, thisinvention provides for the recovery of sodium constituent and available.heat content from the spent-1 115 liquor in the alpha fiber plant,without the installation' of special recovery apparatus.-

I do not herein claim any features eneric lto application, Serial No.72,522, filed ecember 1, 1925, by Milton O. Schur and myself;l .12enor'do I herein cla-imany. features generic to my applications SerialNos. 26,361 and 26,362, both filed April 28, 1925.

What .I'claim is: l -1. A. 'process which lcomprises digesting- 125predigested pulp inl an alkaline liquor containing sodium digesting'vconstituent re ciigred from the spentliquor resulting from theidigestionof cellulose material in the pro",- kraft pulp production, so that theAWhite duction of sulphateor kraft pulp,j ,togethei f 130 Of course, thedigesting.

with the spent liquor resulting from previous digestion of likepredigested pulp with alkaline liquor.

2. A process which comprises digesting predigested pulp in a. freshalkaline liquor containing sodium compounds, mixing sufcient spentliquor from such digestion with spent kraft liquor to make up sodiumlosses suffered in the kraft digesting cycle, and recovering the sodiumconstituent from the spent mixed liquors.

3. An inter-related process of producing high alpha-cellulose pulp andkraft pulp, which comprises digesting predigested pulp in an alkalineliquor containing sodium compounds, re-employing one portion of thespent liquor of the first-mentioned digestion in another digestion oflike predigested pulp, and mixing the .other portion with the spentblack liquor from kraft digestion, recovering the sodium constituent ofthe mixture, and using recovered sodium constituent ina subsequentkraftdigestion and any excess thereof in another digestion ofpredigested pulp.

4. An inter-related process' of producing high alpha-cellulose pulp andkraft pulp, which comprises digesting predigested pulp in an alkalineliquor containing sodium compounds, re-employing one portion of thespent liquor of the first-mentioned digestion in another digestion ofpredigested pulp,

mixing the other portion containing substantially the amount of, spentdigesting agent formed in a preceding digestion with the black liquorresulting from kraft digestion, vrecovering the sodium constituent ofthe mixture,using the recovered sodium constituent necessary for thekraft digestion in. such digestion. and using any excess recoveredsodium constituent in another digestion of predigested pulp, togetherwith the reemployed portion of said first-mentioned spent li'quor.

5. A process which. comprises digesting predigested pulp in an alkalineliquor containing sodium compounds,- mixing one portion of the spentdigesting liquor with the black spent liquor resulting from a kraftdigestion, recovering the sodium constituent thereof, making up lossesin the kraft cycle with the sodium constituent recovered from said oneportion, re-mploying the other por- .tion of the spent liquor in anotherdigestionof predigested pulp, adding any available recovered sodiuln-constituent to said other portion` and making up losses in bothdigesting cycles by the addition of sodium digesting agent to said otherportion.

' 6. A process which comprises digesting predigested'pulp in an alkalineliquor con- -taining sodium compounds, re-employing one portion of thespent liquor in another d1- 4 gestion of pre ested pulp, and mixing theother portion o the/spent liquor containing substantially the amount ofspent digesting agent which was formed in a precedin digestion with theblack liquor resulting rom a kraft digestion, recovering the sodiumconstituent thereof, and mixing a portion of said recovered constituentwith the portion of reemployed spent liquor.

7. A process which comprises digesting predigested pulp in an alkalineliquor containing sodiumv compounds, re-employing substantially all thespent alkaline liquor of the `digestion in another digestion of likepulp after the addition of sufficient sodium digesting agent thereto tomake up losses therein for effective digestion, continuing suchre-employment until a predetermined concentration of sodium resultstherein, and thereafter mixing one portion of the spent sodium alkalineliquor with the black liquor resulting from a kraft digestion,recovering the sodium constituents from the mixed liquor using therecovered sodium constituent in a subsequent kraft digestion, and

rrcs-employing the other portion of spent liquor in a subsequentdigestion of predigested pulp. ,Y

8. A process which comprises digest-ing predigested pulp in ari alkalineliquor containing sodium compounds, re-employing substantially all thespent alkaline liquor of the digestion inanother pulp digestion afterthe addition of suiicient sodium digesting agent thereto to make uplosses therein for effective digestion, continuing such re-employmentuntil a predetermined concentration of sodium results therein, andthereafter re-employing one portion of the spent' sodium alkaline liquorin another digestion of predigested pulp and mixing the other portion ofthe spent liquor with the black liquor resulting from a kraft digestion,rccovering the sodium constituent from the mixed liquors, and using aportion of the recovered sodium constituentfin, a subsequent digestionofpredigested pulp.

9. A .process which' comprises digest-ing predigested pulp in a freshalkaline sodium base liquor, mixing spent liquor from such digestionwith spent liquor resulting from the digestion of cellulosic material inkraft liquor, and recovering the sodium constituentfronithe spent mixedliquors.

10. A continuous cyclic process which comprises continuously feeding anddigesting predigested pulp in an alkaline liquor containing sodiumcompounds, washing the resulting pulp and the entrained products of thereaction, circulating a part of the wash water back for furtherdigestion of predigested pulp, mixing the other part with the blackspent liquor resulting from a kraft digestion, evaporating said mixedliquor, burning and smelting the combustible content thereof in areducing atmosphere, recovering the molten smelt in water, causticizingthe resulting alkaline liquor', re-employing said' liquor in asubsequentkraft digestion, and

employing any excess of said liquor in a subf sequent digestion withsaid recirculated Wash water in the digestion of predigested pulp,

and making up losses in both digesting cycles by the addition of sodiumcompounds to said reirculated Wash Water. y

11. Process comprising the following steps: digesting suliite pulp in analkaline liquor containing sodium sullide to the formation of a pulphaving a high alpha cellulose content and waste alkaline liquor andusing this waste alkaline liquor in the preparation of a cooking liquorfor a sulfate digestion of Wood.

12. The process of making chemical wood pulp having -a high'content ofalpha cellulose involving the digestion of raw cellulose ma- Y terialinan acid sulite cooking liquor, separation of theresultin Waste suliteliquoil from the resulting su lite pulp, and digestion of the sulitepulp in an alkaline cooking liquor, characterized in that the saidalkaline cooking liquor is a solution comprising sodium sulfide obtainedby concentrating smelting and recovering the sodium compounds present inthe waste liquor from a collateral sulfate process of makin pulp.

' 13. The process of vmaing chemical wood pulp having a high content ofalpha cellulose involving the digestion of raw cellulosic material in anacid sulite cooking liquor, separation of Vthe resulting waste sulfteliquor from the resulting sulfite pulp, and digestion i of the sultepulp in an alkaline cooking liquor, characterized in that the saidalkaline cooking liquor is a solution obtained by concentrating andsmelting, in' a reducing atmosphere, the Waste liquor from a collateralsul- 40 fate process of making pulp,-dissolving the resulting smelt inwater, and treating the resulting solution with caustic lime. Intestimony whereof I have affixed my signature.

i5 p GEORGE A. RICHTER.

